The invention is directed to a permanent magnetic structure used to establish a uniform examination field in a nuclear magnetic resonance (NMR) system.
Such systems are well known as magnetic resonance imaging (MRI) instruments for medical imaging of live patients. Actually, such instruments were first developed on a small scale for laboratory use for the examination of inanimate and small animate objects and subsequently on a larger scale for application to human objects. In both applications, usually the higher the intensity of the uniform field in the region of interest, the better the resolution or other output data received from the instrument.
The design of permanent magnetic structures especially for field levels above 1 Tesla (T) faces the problems of saturation of ferromagnetic material as well as the limitations of the remanence of available magnetic alloys.
One category of permanent magnets that is not affected by these limitations is the category of yokeless magnets that are designed to generate and confine the field without the need of external yokes. Because of the linear characteristics of modern rare-earth alloys, such as Nd—B—Fe, or the oriented rare-earth alloys described in Holback, K. “Design of Permanent Multipole Magnets with Oriented Rare-Earth Cobalt Material, “Nuclear Instruments and Methods”, 169, (1980):1–10 (hereinafter “Holback”), yokeless magnets are transparent to the field generated by other sources (see Abele, M. G. “Structures of Permanent Magnets”, New York: John Wiley and Sons, Inc., 1993 (hereinafter “Abele”). Consequently, the field generated within the cavity of a yokeless magnet can be increased by enclosing the magnet within a second magnet and the technique can be extended to a number of concentric magnets to increase the field to levels well above the remanence of the magnetic material, as long as the individual magnets operate within the linear range of their demagnetization characteristics. Of particular importance is the generation of a uniform field within the central cavity.